20 Mile Rock

Idaho Falls, Idaho 83402

Hike and explore a lava/desert playground close to Idaho Falls. The Hell’s Half Acre lava flow is one of several lava flows in Idaho’s Snake River Plain that erupted about 4,100 years ago. It’s 222 square miles, or 162,000 acres.
The loop trail through this unusual terrain is marked by blue-tipped poles. Enjoy the abundant wildflowers such as evening primrose, Indian paintbrush, wild onions, penstemon, geraniums, and prickly pear cactus in May and early June. Since there is no actual trail other than following markers, you can take your time exploring the cracks and crevasses in the lava rock.

20 Mile Rock

1Idaho Falls, Idaho 83402

Hike and explore a lava/desert playground close to Idaho Falls. The Hell’s Half Acre lava flow is one of several lava flows in Idaho’s Snake River Plain that erupted about 4,100 years ago. It’s 222 square miles, or 162,000 acres.

The loop trail through this unusual terrain is marked by blue-tipped poles. Enjoy the abundant wildflowers such as evening primrose, Indian paintbrush, wild onions, penstemon, geraniums, and prickly pear cactus in May and early June. Since there is no actual trail other than following markers, you can take your time exploring the cracks and crevasses in the lava rock.

20 Mile Rock Professional Guide

Detailed Trail Description from our Guidebook

"Hike and explore a lava/desert playground close to Idaho Falls. The Hell’s Half Acre lava flow is one of several lava flows in Idaho’s Snake River Plain that erupted about 4,100 years ago. It’s 222 square miles, or 162,000 acres.

The loop trail through this unusual terrain is marked by blue-tipped poles. Enjoy the abundant wildflowers such as evening primrose, Indian paintbrush, wild onions, penstemon, geraniums, and prickly pear cactus in May and early June. Since there is no actual trail other than following markers, you can take your time exploring the cracks and crevasses in the lava rock."